1. Field of the Invention
The present invention relates to a fuel gas production system that produces hydrogen, which is to be fed to fuel cells, from a raw material containing hydrogen atoms. More specifically the present invention pertains to a fuel gas production system having a hydrogen separation mechanism that separates hydrogen in the course of production.
2. Description of the Related Art
Each of the fuel cells has a hydrogen electrode and an oxygen electrode disposed across an electrolyte layer, which hydrogen ions pass through, and generates an electromotive force through the following reactions proceeding at the respective electrodes:
Hydrogen electrode: H2xe2x86x922H++2exe2x88x92
Oxygen electrode: (xc2xd)O2+2H++2exe2x88x92xe2x86x92H2O
A hydrogen-rich fuel gas may be produced by reforming a hydrocarbon compound, such as methanol or natural gas, in a fuel gas production system. The material is decomposed to the hydrogen-rich fuel gas stepwise through plural stages of reactions in the fuel gas production system.
The first stage reaction is called the reforming reaction and is expressed by Equations (1) and (2) given below in the case of a hydrocarbon material
CnHm+nH2Oxe2x86x92nCO+(n+m/2)H2xe2x80x83xe2x80x83(1)
CnHm+n/2O2xe2x86x92nCO+m/2H2xe2x80x83xe2x80x83(2)
The second stage reaction utilizes steam to oxidize carbon monoxide produced by the reforming reaction while producing hydrogen. This second stage reaction is called the shift reaction and is expressed by Equation (3) given below:
CO+H2Oxe2x86x92CO2+H2xe2x80x83xe2x80x83(3)
In some cases, the CO oxidation is subsequently performed as the third stage reaction. The CO oxidation selectively oxidizes carbon monoxide that has not been oxidized by the shift reaction but remains. Carbon monoxide contained in the fuel gas may poison the electrodes and interfere with the stable reactions. The shift reaction and the subsequent CO oxidation sufficiently lower the concentration of carbon monoxide, so as to prevent the potential poisoning of the electrodes.
One application utilizes hydrogen separated from the gaseous mixture, which has been produced through the chemical reactions, as the gaseous mixture. The separation of hydrogen enhances the hydrogen partial pressure in the fuel gas and effectively prevents the gaseous mixture from containing any noxious components.
FIG. 36 schematically illustrates the structure of a prior art fuel cells system including a hydrogen separation mechanism. Supplies of a material and water are respectively fed from a material reservoir 200 and a water reservoir 230 to a reformer unit 216 via an evaporator 212. A reforming reaction proceeds in the reformer unit 216 to produce a gaseous mixture including carbon monoxide. Gaseous hydrogen included in the gaseous mixture permeates a hydrogen separation membrane 218 to a separation unit 220. For the purpose of efficient separation of hydrogen, a gas for carrying out hydrogen (hereinafter referred to as the purge gas) is introduced into the separation unit 220. The purge gas used here is steam obtained by evaporating water led from the water reservoir 230 by an evaporator 232. The separated hydrogen is supplied to fuel cells 228 after removal of the excess water content in a condenser 226. The gaseous mixture after separation of hydrogen (hereinafter referred to as the residual gas) includes carbon monoxide and remaining hydrogen that has not been separated by the hydrogen separation membrane 218. The residual gas is discharged to the outside after carbon monoxide and hydrogen included therein are oxidized in a combustion unit 222.
Any of a variety of condensable gases that have no adverse effects on the fuel cells may be used for the purge gas, in place of steam. A substance that has a small heat of vaporization and is liquid at ordinary temperature is generally suitable for the purge gas. FIG. 37 is a graph showing the relationship between the heat of vaporization and the boiling point. This is cited from the Chemical Handbook. According to the above conditions, paraffin hydrocarbons, dimethyl ether, and acetic acid are suitable for the purge gas.
The prior art fuel gas production system, however, has several problems discussed below.
The prior art technique has an insufficient production efficiency of hydrogen from the material and a relatively low hydrogen partial pressure in the fuel gas. For example, part of the material is not subjected to any reaction but is discharged to the outside. In the fuel gas production system having the hydrogen separation mechanism, hydrogen remaining in the residual gas is often wasted. The low production efficiency of hydrogen leads to an increase in consumption of the material. This results in raising the operation cost, increasing the required capacity of the material reservoir, and thereby expanding the size of the whole fuel cells system. The low hydrogen partial pressure in the fuel gas results in lowering the efficiency of power generation of the fuel cells and thereby expanding the size of the whole fuel cells system.
The prior art fuel gas production system requires the evaporator and the water reservoir for producing the purge gas. This leads to the size expansion and the complicated structure of the fuel gas production system.
In the prior art fuel gas production system, the flow rate of the purge gas is substantially not regulated. The flow rate of the purge gas affects the separation efficiency of hydrogen and the driving efficiency of the fuel cells. Such effects are especially prominent when the driving conditions of the fuel cells change. The insufficient flow rate of the purge gas may cause an insufficient supply or a delayed supply of the fuel gas and a response delay of the hydrogen output.
From the viewpoint of the environmental protection, the recent requirement is to mount such fuel cells on a vehicle. For this purpose, the lowered driving efficiency and the size expansion of the whole fuel cells system are significant problems.
The object of the present invention is accordingly to enhance the production efficiency of hydrogen and raise the hydrogen partial pressure in a resulting fuel gas in a fuel gas production system, to enable efficient production of a purge gas, and to appropriately regulate the flow rate of the purge gas to improve the driving efficiency and the response of fuel cells.
At least one of the above and the other related objects is actualized by a first fuel gas production system that produces a hydrogen-rich fuel gas, which is to be supplied to fuel cells, from a raw material. The fuel gas production system includes: a chemical reaction device that produces a gaseous mixture containing hydrogen from the raw material through a plurality of chemical processes; a hydrogen separation mechanism that separates hydrogen from the gaseous mixture in at least one place of the chemical reaction device; and a flow path that feeds both the hydrogen separated by the hydrogen separation mechanism and a residual gas after the separation of hydrogen from the gaseous mixture to the fuel cells, so as to ensure a supply of all hydrogen obtained in all the chemical processes in the chemical reaction device to the fuel cells.
Any of a variety of compounds containing hydrogen atoms, for example, hydrocarbons like gasoline, alcohols, ethers, and aldehydes, may be used for the raw material.
The flow path is arranged to supply both the hydrogen separation by the hydrogen separation mechanism and the residual gas to the fuel cells. This arrangement enables not only the separated hydrogen but the remaining hydrogen, which has not been separated from the gaseous mixture by the hydrogen separation mechanism, to be supplied to the fuel cells. Namely the arrangement of the present invention enables most of the hydrogen produced through the plurality of chemical processes to be supplied to the fuel cells. The separation of hydrogen by the hydrogen separation mechanism enhances the hydrogen partial pressure in the resulting fuel gas. The separation also accelerates the reaction proceeding in the chemical reaction device. Introduction of a purge gas improves the separation efficiency of hydrogen and enables efficient extraction of hydrogen from the chemical reaction device, so as to lower the concentration of hydrogen in the chemical reaction device. The reaction in the chemical reaction device is reversible and the rate of the reaction is affected by the concentration of hydrogen. The lowered concentration of hydrogen in the chemical reaction device accordingly enhances the rate of reaction.
In accordance with one preferable application of the fuel gas production system where the chemical reaction device has a plurality of reaction units, the hydrogen separation mechanism may be disposed at a specific side other than a last reaction unit. In this application, the flow path is constructed to ensure a supply of both the hydrogen separated by the hydrogen separation mechanism and hydrogen produced after the hydrogen separation The chemical process of producing hydrogen is the reversible reaction, so that using the residual gas having the low hydrogen partial pressure accelerates the reaction and enhances the production efficiency of hydrogen. This arrangement does not waste the residual gas after the separation of hydrogen but utilizes it for subsequent production of hydrogen. This advantageously saves the material.
In one concrete example where the chemical reaction device includes a reformer unit for a reforming reaction and a shift unit for a shift reaction , the hydrogen separation mechanism is located before the shift unit. This arrangement accelerates the reaction in the shift unit.
The hydrogen separation mechanism may be integrated with or separate from any reaction unit of the chemical reaction device. The hydrogen separation mechanism may be provided at a plurality of sides, for example, in the respective reaction units of the chemical reaction device.
The hydrogen separation mechanism may include a hydrogen separation membrane that has selective permeability to hydrogen. The hydrogen separation membrane is arranged to have opposing two faces, that is, a feeding face and an extraction face. The former receives a supply of the gaseous mixture and the latter extracts selectively permeating hydrogen from the gaseous mixture. In this case, it is desirable to introduce a flow of a purge gas for carrying out the hydrogen to the extraction face.
The rate of hydrogen permeation through the hydrogen separation membrane depends upon the difference in hydrogen partial pressure between the feeding face and the extraction face. The active carriage of hydrogen on the flow of the purge gas lowers the hydrogen partial pressure on the extraction face, so as to increase the difference in hydrogen partial pressure and enhance the rate of hydrogen permeation.
The flow of the purge gas may be introduced under conditions that a hydrogen partial pressure on the feeding face is higher than that on the extraction face thereof and that a total pressure on the feeding face is even or lower than that on the extraction face.
Even when there is a pinhole in the hydrogen separation membrane, the difference in total pressure effectively prevents a carbon monoxide-containing gas from leaking from the feeding face to the extraction face. This arrangement enables advantageous reduction in thickness without any fear of potential troubles caused by the presence of pinholes. In the case where steam is used for the purge gas, the steam permeating from the extraction face to the feeding face due to the difference in total pressure undergoes the reforming reaction and the shift reaction. The structure using the vaporized material hydrocarbon as the purge gas also ensures the similar advantages to those of the structure using the steam as the purge gas.
In order to keep the hydrogen partial pressure at a relatively low level on the extraction face and ensure the efficient separation of hydrogen, it is desirable to regulate the flow rate of the purge gas according to the flow rate of the material gas. The regulation of the flow rate is attained by electronically controlling the on-off state of a valve. In accordance with one preferable embodiment, a gas flow rate regulation mechanism may be provided in the flow path to automatically vary the flow rate of the purge gas while holding a predetermined correlation with the flow rate of the residual gas. There is a fixed correlation between the flow rate of the material gas and the flow rate of the residual gas. By taking advantage of such correlations, the flow rate of the purge gas is regulated according to the flow rate of the material gas by the simple structure. This arrangement advantageously reduces the size and the manufacturing cost of the whole fuel gas production system.
A jet pump may be used for the gas flow rate regulation mechanism. The jet pump has two flow-in systems and one flow-out system. A negative pressure is generated in the jet pump by a flow of a high-pressure fluid into one flow-in system, and another fluid is thereby sucked into the other flow-in system. The fluids of the two systems flow at specific flow rates holding a fixed correlation. In the fuel gas production system of the present invention, the flow of the hydrogen separated by the hydrogen separation mechanism joins with the flow of the residual gas and is then supplied to the fuel cells. The jet pump may be located at this meeting point. The jet pump does not have any mechanical movable part and accordingly has high reliability.
A second fuel gas production system of the present invention has a noxious component reduction unit where the residual gas after the separation of hydrogen undergoes a reduction process, which reduces the concentration of a noxious component that is harmful to the fuel cells. The residual gas after the reduction process is used as the purge gas.
As discussed in the prior art, a condenser and an evaporator are required in the structure using the steam as the purge gas. The structure using the residual gas as the purge gas, on the other hand, enables size reduction or even omission of the condenser and the evaporator. This advantageously reduces the size of the whole fuel gas production system.
The structure using the residual gas as the purge gas enables the hydrogen in the residual gas to be supplied to the fuel cells. This arrangement advantageously ensures the efficient use of hydrogen.
The reduction process may be an oxidation or a catalytic reaction of the residual gas. One typical example of the oxidation is combustion. Typical examples of the catalytic reaction include the selective oxidation of carbon monoxide and the shift reaction. Such reactions of the reduction process effectively lower the concentration of carbon monoxide. The reduction process is, however, not restricted to these examples but may be selected appropriately to reduce the concentration of the noxious component that adversely affects the fuel cells. The reduction process may include a plurality of reactions.
In the fuel gas production system of the present invention, in addition to the residual gas, an off gas of the fuel cells may be used as the purge gas. This arrangement ensures a sufficient flow of the purge gas even when the residual gas does not have a sufficient amount for the purge gas.
In the structure that uses the anode off gas discharged from the anodes as the purge gas, hydrogen in the anode off gas, which has not been utilized for power generation, is supplied again to the fuel cells. This advantageously ensures the efficient use of hydrogen. In the structure that uses the cathode off gas discharged from the cathodes as the purge gas, no content of hydrogen in the cathode off gas advantageously prevents an increase in hydrogen partial pressure on the extraction face and thereby enhances the separation efficiency of hydrogen.
The flow of the off gas may be introduced to any side that allows the use of the off gas as the purge gas, that is, any side on the upper stream side of the extraction face.
The first side of introducing the flow of the off gas is the upper stream side of the chemical reaction device.
The second side of introducing the flow of the off gas is between the feeding face and the noxious component reduction unit.
The third side of introducing the flow of the off gas is between the noxious component reduction unit and the extraction face.
In the structure that introduces the flow of the off gas to the upper stream side of the chemical reaction device (that is, the first side), the components of the off gas undergo the chemical reaction proceeding in the chemical reaction device. For example, in the case where the flow of the cathode off gas is introduced to the first side, a trace amount of remaining oxygen in the cathode off gas is subjected to the reforming reaction (shown by Equation (2) given previously). Oxygen in the purge gas is accordingly consumed in the chemical reaction device. This effectively prevents hydrogen in the resulting fuel gas from being wastefully consumed by the reaction of the hydrogen extracted through the hydrogen separation membrane with the trace amount of oxygen in the purge gas. The cathode off gas has a low hydrogen partial pressure, so that the introduction of the flow of the cathode off gas advantageously enhances the rate of the reaction proceeding in the chemical reaction device. In the case where the flow of the anode off gas is introduced to the first side, on the other hand, water produced through the reaction in the fuel cells is subjected to the reforming reaction (shown by Equation (1) given previously).
The structure that introduces the flow of the off gas to the position between the feeding face and the noxious component reduction unit (that is, the second side) has the following advantages. In the case where the flow of the cathode off gas is introduced to the second side, remaining oxygen in the cathode off gas is subjected to the oxidation of the reduction process. Like the introduction to the first side, the introduction of the cathode off gas to the second side also has the advantages regarding the consumption of oxygen and the hydrogen partial pressure. In the case where the flow of the anode off gas is introduced to the second side, on the other hand, hydrogen in the anode off gas is oxidized to steam and the anode off gas having the lowered hydrogen partial pressure is led into the extraction face This arrangement effectively enhances the separation efficiency of hydrogen.
The structure that introduces the flow of the off gas to the position between the noxious component reduction unit and the extraction face (that is, the third side) has the following advantages. In the case where the flow of the cathode off gas is introduced to the third side, the gas having the low hydrogen partial pressure is used as the purge gas. This arrangement effectively enhances the separation efficiency of hydrogen. In the case where the flow of the anode off gas is introduced to the third side, on the other hand, the remaining hydrogen in the anode off gas is reused in the fuel cells. Another advantage of this structure using either the flow of the cathode off gas or the flow of the anode off gas is that the temperature of the purge gas is sufficiently close to the driving temperature of the fuel cells. The reaction proceeds at extremely high temperatures in the chemical reaction device. The temperature of the fuel gas should thus be lowered by a heat exchange unit, prior to the supply to the fuel cells. The introduction of the off gas to the third side enables extraction of the hydrogen with the purge gas having the temperature sufficiently close to the driving temperature of the fuel cells and thereby makes the temperature of the supply of the fuel gas sufficiently close to the driving temperature of the fuel cells. This arrangement advantageously enables size reduction or even omission of the heat exchange unit.
A third fuel gas production system of the present invention utilizes the flow of the cathode off gas as the purge gas.
The cathode off gas has the hydrogen partial pressure substantially equal to zero and is thus suitable for the purge gas. The enhanced utilization efficiency of oxygen on the cathodes reduces the quantity of oxygen included in the cathode off gas to a trace level and thus substantially prevents the reaction of oxygen with the extracted hydrogen. Another advantage of this arrangement is that the temperature of the purge gas is sufficiently close to the driving temperature of the fuel cells.
A fourth fuel gas production system of the present invention has a circulation mechanism that circulates the flow of the anode off gas to the extraction face as the purge gas. This arrangement enables the remaining hydrogen in the anode off gas to be reused in the fuel cells.
In accordance with another preferable application of the present invention, the purge gas for carrying out the hydrogen may be a processed gas after a reduction process that reduces the concentration of at least one of hydrogen and a specific component, which has high reactivity to hydrogen, included in the gas prior to supply to the fuel cells. The reduction process of hydrogen includes consumption of hydrogen in the fuel cells and combustion of hydrogen. The specific component having high reactivity to hydrogen may be carbon monoxide and oxygen. The reduction process of oxygen includes consumption of oxygen through oxidation or combustion.
It is desirable to regulate the flow rate of the purge gas in any of the second through the fourth fuel gas production systems. In accordance with one preferable application, the relationship between the a load on the fuel cells and the flow rate of the purge gas is stored in advance, and the flow rate of the purge gas is regulated according to the observed a load on the fuel cells. The relationship between the a load on the fuel cells and the flow rate of the purge gas may be stored in any suitable form, for example, in the form of a table or in the form of a function.
The flow rate of the purge gas affects the separation efficiency of hydrogen and thereby the driving efficiency of the fuel cells. Regulation of the flow rate of the purge gas according to the a load on the fuel cells thus enhances the driving efficiency of the fuel cells. The relationship between the a load on the fuel cells and the flow rate of the purge gas is set experimentally or analytically by taking into account the driving efficiency of the fuel cells. The following two points should especially be considered for the setting: the required quantity of hydrogen according to the a load on the fuel cells and the energy loss due to an increase in flow rate of the purge gas. When the fuel cells are in a high loading state, it is desirable to increase the flow rate of the purge gas and enhance the separation efficiency of hydrogen, in order to allow a supply of a large quantity of hydrogen. The increase in flow rate of the purge gas, on the other hand, results in increasing the power required for the introduction of the purge gas and thus enhancing the energy loss. Consideration of these two affecting factors according to the a load on the fuel cells ensures the appropriate setting of the flow rate of the purge gas to attain the optimum energy efficiency. The arrangement of regulating the flow rate of the purge gas is applicable to the structure that uses only the anode off gas as the flow of the purge gas.
In the case where the a load on the fuel cells increases by a rate of change of not less than a predetermined level, it is preferable to significantly increase the flow rate of the purge gas than the usual setting, that is, to correct a preset value, in the process of regulating the flow rate of the purge gas. Here the usual setting means a preset flow rate according to the a load on the fuel cells. The correction of the flow rate may be set in a variety of forms, for example, by a function of the rate of change or as a preset value.
The increased flow rate of the purge gas enhances the flow velocity of the whole fuel gas and thereby improves the dispersibility of the fuel gas in the fuel cells. The improved dispersibility results in enhancing the utilization rate of hydrogen in the fuel cells. The increased flow rate of the purge gas also improves the separation efficiency of hydrogen. This leads to an increase in quantity of hydrogen present in the flow path of carrying the fuel gas from the lower stream side of the hydrogen separation mechanism to the fuel cells. When there is any response delay in the course of producing hydrogen in the chemical reaction device or in the course of separating hydrogen by the hydrogen separation mechanism, the hydrogen present in the flow path works to compensate for the response delay. Because of such functions, in the case of an abrupt increase in a load on the fuel cells, the regulation of the flow rate of the purge gas according to the rate of change in loading state ensures output of the required electric power with the high response.
The arrangement of regulating the flow rate of the purge gas according to the rate of change in a load on the fuel cells may be applied to the structure that flows the purge gas at a fixed flow rate in the standard conditions. The regulation of the flow rate by considering the rate of change in loading state is also applicable to a variety of structures. For example, in the case of an abrupt decrease in a load on the fuel cells, the regulation may lower the flow rate significantly.
One modified structure has an additional gas flow source, in addition to the flow of the purge gas mainly used. In the case of an insufficient flow rate of the purge gas, this additional gas flow source is activated.
This arrangement facilitates the maintenance of the required flow rate of the purge gas according to the a load on the fuel cells. A gas source that does not require any special reservoir, for example, the air, may be used for the additional gas flow source.
The fuel gas production system of the above configuration may feed a supply of the fuel gas to the fuel cells directly, but more preferably via gas-liquid separation mechanism that separates steam from the fuel gas. The anodes of the fuel cells are generally moistened to accelerate the shift of the hydrogen ion. When the excess steam is present, however, there is a possibility that the inside of the electrodes sweats to lower the efficiency of power generation. The arrangement of supplying the fuel gas to the fuel cells after the separation of steam effectively prevents such potential troubles.
In the fuel gas production system discussed above, when an oxygen-containing gas, for example, the air, gaseous oxygen, or the cathode off gas, is used as the flow of the purge gas, it is desirable to increase the quantity of the oxygen-containing gas when the fuel cells have not yet been warmed up. This arrangement accelerates the warm-up of the fuel cells by utilizing the heat of the reaction of oxygen in the oxygen-containing gas with hydrogen in the fuel gas. In the structure of circulating the anode off gas, the circulation may be interrupted during the warm-up operation of the fuel cells. When the oxygen-containing gas is used as the flow of the purge gas, substantially no hydrogen remains in the anode off gas. The circulation of the anode off gas accordingly does not lead to the effective use of hydrogen. This arrangement is actualized by a mechanism of switching over the flow path of the anode off gas between the circulation to the hydrogen separation mechanism and the discharge to the outside.
The principle of the present invention is attained by a variety of applications other than the fuel gas production system discussed above. One possible application is a fuel cells system including the fuel gas production system discussed above. Other possible applications include a method of producing a fuel gas for fuel cells and a method of separating hydrogen from a gaseous mixture as one step in the fuel gas producing method. As mentioned above, the fuel cells may be warmed up by regulating the quantity of the air mixed with the purge gas. From this point of view, the present invention may be directed to a method of warming up fuel cells by utilizing the flow of the fuel gas.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.